Ionization-type fire detectors are well known in the art. It is common in such detectors to provide a pair of ionization chambers which are constantly maintained ionized by a source of radiation. One of the chambers is so arranged that the products of combustion can enter therein and thus affect the electrical conductivity in the chamber, whereas the other chamber is generally substantially isolated from the products of combustion and thus has its conductivity affected only negligibly, if at all, thereby. An electronic bridge and amplifier circuit responds to the conductivity levels in the two separate chambers and senses their difference in conductivity when combustion products enter the first chamber, being then effective to provide a distinctive alarm signal.
One of the disadvantages of the prior art ionization-type fire detectors is that they are generally quite bulky and decidedly unattractive in appearance. Another drawback is their considerable susceptibility to drafts of wind. More particularly, it has been found that a draft of wind or strong air current passing through the first-mentioned ionization chamber tends to affect the electrical conductivity of that chamber in much the same way that occurs when products of combustion enter the chamber, and it may thus occur therefore that a false fire alarm signal may be given when such wind drafts or air currents are present.